Radiofrequency (RF) electrode catheters have been in common use in medical practice for many years. They are used to stimulate and map electrical activity in the heart and to ablate sites of aberrant electrical activity. Specifically, targeted ablation may be performed for a number of indications. For example, ablation of myocardial tissue is well known as a treatment for cardiac arrhythmias by using a catheter to apply RF energy and create a lesion to break arrhythmogenic current paths in the cardiac tissue. As another example, a renal ablation procedure may involve the insertion of a catheter having an electrode at its distal end into a renal artery in order to complete a circumferential lesion in the artery in order to denervate the artery for the treatment of hypertension. More generally, RF energy may be delivered to a treatment site within a patient's body to ablate tissue and/or to form a lesion.
In such procedures, a reference electrode is typically provided and may be attached to the skin of the patient or by means of a second catheter. RF current is applied between the tip electrode of the ablating catheter and the reference electrode, and current flows through the media that surrounds them, i.e., blood and tissue. The distribution of current from the ablation tip electrode depends on the amount of electrode surface in contact with the tissue as compared to blood, which has a higher conductivity than the tissue. Heating of the tissue occurs due to its electrical resistance. The tissue is heated to a desired degree, such as an amount sufficient to cause cellular destruction in the target tissue resulting in formation of a lesion which is electrically non-conductive. The lesion may be formed in tissue contacting the electrode or in adjacent tissue.
Although such techniques involve the intentional heating of tissue in order to effect a change in the patient's physiology, it is desirable to control the amount of energy delivered to reduce collateral damage to tissue surrounding the treatment area. Notably, during RF ablation, the delivery of energy may be sufficient to vaporize water present at the treatment site, resulting in a phenomenon known as a “steam pop.” The rapid increase in volume associated with the transition of water from liquid phase to gas phase transmits force to the surrounding tissue and has the potential to cause adverse results, such as thrombus formation, perforation of tissue and/or tamponade.
To reduce the occurrence of these and other negative consequences of a steam pop, the attending electrophysiologist typically ceases delivery of RF energy to the current treatment site when a steam pop is audible. However, it will be appreciated that this conventional practice does not represent an optimum technique for controlling the delivery of energy during an ablation procedure. For example, steam pops may occur during a procedure that are inaudible for a variety of reasons, such as location of the treatment site or magnitude of the pop. Further, even if a resulting steam pop is audible, there may be a delay in the reaction time of the electrophysiologist, so that RF energy continues to be delivered for a period of time after detection of the pop. Under these or other circumstances, RF energy may continue to be delivered despite the occurrence of steam pops, leading to an increased risk of the noted adverse events.
Accordingly, it would be desirable to provide methods and systems for improving the detection of steam pops during an RF ablation procedure. Similarly, it would be desirable to reduce the amount of time required to cease delivery of RF energy or otherwise adjust the ablation after a steam pop is detected. As will be described in the following materials, this disclosure satisfies these and other needs.